Powered lift aircraft, aircraft in which during flight a part of the sustaining lift is derived from the propulsion system, are well-known in the art. Such aircraft depend on the use of high thrust levels to generate the lift required for short distance (STOL) landings. In order for an aircraft to descend, the total force on the aircraft along an axis in the direction of the relative wind must be positive; that is, the drag must be greater than the thrust along such axis. In powered lift aircraft, the necessity for the drag to be greater than the thrust for the aircraft to descend presents a problem since it is also desirable to make maximum use of the thrust provided by the propulsion system to generate lift, which of course must always equal the weight of the aircraft when the aircraft is in the air.
Known methods and apparatus for accomplishing descent and short distance landing of powered lift aircraft when used in connection with aircraft which have jet-flap type high lift systems (which include internally blown jet flaps, augmentor wings, ejector flaps, externally blown flaps, and upper surface blowing) have been unable to take full advantage of all of the thrust provided by the propulsion system to generate lift since the aircraft could not descend when the propulsion system was set at maximum thrust. This inability to descend at full thrust is due to the phenomenon known as "jet flap thrust recovery". Suction is developed on the wing or airfoil leading edge. This suction is approximately equal to the thrust at the wing trailing edge, and its effect is to decrease the drag on the aircraft and thereby make descent more difficult. Theoretically, the suction developed is independent of the downward deflection of jet flow at the trailing edge to produce lift.
There have been a number of previous attempts to solve the problem of providing satisfactory descent performance of powered lift aircraft with jet-flap type high lift systems. These attempts have centered on reducing thrust operation of the propulsion system or decreasing the efficiency of the lift-generating jet flow deflection process at high jet deflection angles. Both of these approaches result in a relative loss in lift since the lift is less than that which could be obtained if all of the installed thrust were used efficiently to provide lift. The net result is that the slowing of the speed of the aircraft during descent is not maximized and therefore the landing distance is not minimized.